Space frame architecture is increasingly being used in vehicle manufacturing and represents a relatively new approach to vehicle construction. A space frame is an assembly of individual frame components that are connected at joints to form a cage-like structure on which the other vehicle components can be mounted such as the engine, drive train, suspension and the hang-on vehicle body parts. The hang-on vehicle body parts may include the floor pan, roof, fenders, doors, body panels, hood and trunk lid. Conventional space frames have typically been constructed from numerous stamped or roll-formed parts which are welded or otherwise joined together. In the automotive industry there is always a continuing need to reduce the weight and number of parts in order to produce more cost-effective and fuel efficient vehicles. At the same time, it is important to maintain the structural integrity of the vehicle structure.
The present invention stems from the recognition that stamped and roll formed vehicle space frame members require numerous assembly steps and joints that may result in significant tolerance build-up.
Tubular hydroforming offers many advantages in space frame construction because it can enable manufacturers to better control frame stiffness, dimensional stability, fatigue life, and crash worthiness over prior vehicle designs while reducing frame mass and cost. Hydroforming is a metal-forming process in which high pressure fluid is used to outwardly expand a tubular blank into conformity with surfaces of a die cavity of a die assembly to form an irregularly shaped tubular part. Hydroformed members can be provided with a wider range of geometries in comparison with stamped or roll formed parts. Each frame member can have a cross-sectional configuration that varies continuously along its length, to the configuration desired.
As a result a single hydroformed part can often replace a plurality of stamped (or roll formed) and welded parts, thereby reducing the number of parts necessary to complete frame construction. Consequently, vehicle weight and assembly cost are reduced.
Hydroformed parts are also advantageous because they have a higher strength than stamped parts, primarily because of the plastic deformation of the wall of the blank during the hydroforming process. More particularly, the outward expansion of the wall of the blank during hydroforming caused by the fluid pressure creates a work-hardening effect which uniformly hardens the metal material of the blank. Hydroforming also produces less waste metal material than stamping.
Thus, tubular hydroforming has many advantages over more conventional stamping and welding technologies. For the promise of hydroforming technology to be fully realized in the creation of vehicle space frames, the individual hydroformed members that make up the space frame must be able to be quickly and accurately assembled into space frames for mass production. This requires that joints between individual hydroformed members be quickly and easily formed. Typical space frame designs utilize separate joint forming structures, sometimes called "nodes", to connect two or more elongated hydroformed members. An example of a conventional node having several short discrete projections extending outwardly at predetermined angles and of the use of nodal architecture in space frame construction is shown in WO 97/00595.
Nodal architecture of the type shown in WO 97/00595 requires the elongated hydroformed frame members to be assembled together in a fixed, predetermined angular and spatial arrangement. Each outward projection on a '595 node is telescopically interengaged with and then affixed to an opening in one of the elongated tubular hydroformed members to be joined. The end of the each hydroformed member is usually affixed to the node by welding.
Nodal space frame architecture presents manufacturing difficulties that limit the commercial feasibility of the space frames constructed using this design concept. The nodes themselves are difficult to manufacture. It may not be easy to form nodes by hydroforming a tubular blank (depending on the node size, geometry and so on). For this reason, nodes are sometimes formed by methods other than hydroforming such as by casting. The use of nodes in space frame construction also requires a relatively large number of welds to form a single joint. For example, if three elongated hydroformed members are to be joined with one node, three welds are required. Such connections add to the tolerance stack-up and detract from the dimensional reproduceability from frame to frame. In addition, use of nodes adds to the total number of parts required to assemble the space frame, thus also adding to the total weight and cost of the frame.
For manufacturers to be able to mass produce hydroformed space frames in a time and cost effective manner, there is a need for the joints between the individual members to be quickly and accurately formed with a minimum number of parts and a minimum number of welds. Consequently, there is a need for a method of joining hydroformed members together directly without the use of separate nodes to reduce the number of parts and welds.